The present invention relates to a method of operating a gasdynamic CO.sub.2 -laser. Such lasers have a Laval nozzle in which the lasing medium is cooled due to the expansion downstream of the Laval nozzle. Due to such cooling of the lasing medium by expansion with the aid of the Laval nozzle, the so-called translation temperature is lowered so quickly that the vibration or oscillation energy stored in the N.sub.2 -molecule is subject to a so-called "freeze up". In order to decouple or extract this energy in a resonator as radiation energy, it is necessary to reduce the translation temperature to at least room temperature. This reduction of the translation temperature is possible in a gasdynamic CO.sub.2 -laser having a Laval nozzle if the stagnation temperature is within the range of 1500.degree. to 1800.degree. K.
However, the energy available for decoupling relative to the mass throughput increases substantially and more than proportional with reference to a rising stagnation temperature. Therefore it is desirable to operate such lasers at the maximally possible stagnation temperature. This desirability of keeping the stagnation temperatures at a maximum, poses the problem of sufficiently lowering the translation temperature during the expansion process. For this purpose it is necessary to increase the surface area ratio F/F*, wherein F* is the cross-sectional surface area of the nozzle. However, at large surface area ratios the mach numbers are high and the Laval nozzle becomes ineffective. At large mach numbers the ratio of T.sub.1 /T.sub.2 approaches a limit value as follows. EQU T.sub.1 /T.sub.2 =(F.sub.2 /F.sub.1).sup..gamma.-1 ;
wherein T.sub.1 and T.sub.2 are translation temperatures at different cross-sectional nozzle areas and wherein F.sub.1 and F.sub.2 are these different nozzle cross-sectional areas at two different points along a flow channel, and wherein .gamma. is the adiabatic coefficient. This value or coefficient may be, for example, approximately 1.3 for CO.sub.2 -N.sub.2 -H.sub.2 O lasers.